This invention relates to a medical device for the noninvasive location and imaging of peripheral nerves. Specifically, the present invention is a sensor system for use at the skin surface comprising an electrode array assembly with one or more electrodes and a sensor attachment system. Each electrode in the electrode array assembly maintains a connection to peripheral nerve detection and imaging instrumentation. One or more return wires are attached to the electrode array assembly and to a skin surface electrode during use of the sensor system. A disposable, sterile sensor attachment system allows conductance between the electrode array and the skin surface of the subject. The sensor attachment system contains individual conductor islands, each adapted to align accurately with a specific electrode of the electrode array. The layer of the sensor attachment system that adheres to the skin surface of the subject may be left on the skin at the end of sampling to provide a skin marking guide. This facilitates the positioning of needles for subsequent nerve stimulation or therapy.
The use of direct current skin surface conductance or resistance measurements has been employed in many forms for the identification of peripheral nerves, myofascial trigger points, and acupuncture points (Cory, et al., Characterization of cutaneous electrical hyperconductivity sites associated with myfascial trigger points and tarsal tunnel syndrome. In: Abstracts, 8th World Congress on Pain (Seattle: IASP Press, 1996), p. 54; Kaslow A L and Lowenschuss O. Dragon chasing: a new technique for acupuncture point finding and stimulation. American Journal of Acupunture, 3(2):157-160, 1975); Kejariwal, et al., Development of a device for non-invasive nerve location. In: Abstracts, 8th World Congress on Pain (Seattle: IASP Press, 1996), p.279-280; Kwok, et al., Mapping acupunture points using multi channel device. Australas Phys Eng Sci Med, 21(2):68-72, 1998; Lykken, Square-wave analysis of skin impedance. Psychophysiology, 7(2):262-275, 1971. An early example of this was the use of a transcutaneous electrical nerve stimulation (TENS) unit to identify acupuncture points. When a TENS unit is coupled between examiner and subject, the finger of the examiner acts as a sampling electrode (Kaslow, et al., Dragon chasing: a new technique for acupuncture point finding and stimulation. American Journal of Acupunture, 3(2):157-160, 1975)). However, the literature in the field illustrates inconsistency in locating these sites through electrical conductance measurements (Reichmanis et al., Electrical correlates of acupuncture points. IEEE Transactions on Biomedical Engineering, BME 22:533-535, 1975).
U.S. Pat. No. 4,016,870 to Lock describes a system for acupuncture point location in which a single, hand-held probe of undisclosed composition is used to determine sites of high skin surface conductance. U.S. Pat. No. 5,897,505 to Feinberg, et al., describes a system for measuring selective tissue conductance and temperature utilizing chrome-plated, brass electrodes in a handheld embodiment. Although each of these systems measures conductance at the skin surface, they suffer two main drawbacks. First, metallic electrodes display uneven current densities at the skin surface-electrode interface, which is largely dependent on the underlying moisture pattern of the skin. Devices for measuring skin surface conductance and resistance that do not employ aqueous interfaces are particularly subject to this effect and, in some cases, current densities became high enough to produce a painful sensation. Second, handheld devices are subject to uncontrolled application pressures. This is complicated in larger diameter electrode systems, such as that of U.S. Pat. No. 5,897,505 to Feinberg, where the angle of application causes pressure to be unequally distributed on the skin surface. The use of electrical conductance measurements at the skin surface to locate nerve tissue is facilitated by the use of aqueous electrodes, rather than metallic or dry silverxe2x80x94silver chloride electrodes, and by the use of non-sinusoidal, alternating current waveforms. Based upon observations such as these, a device that locates peripheral nerves transcutaneously was disclosed in the commonly owned U.S. Pat. No. 5,560,372 to Cory (the disclosure of which is incorporated herein by reference).
FIG. 9 is a circuit diagram of the non-invasive, peripheral nerve mapping device according to the U.S. Pat. No. 5,560,372 to Cory as it is positioned over the forearm of a patient. The sampling electrode (10) depicted herein comprises eight electrodes (10a-h) having leads (41) arranged in a linear array and applied to the volar surface of the forearm on the epidermal surface (80). The reference electrode (70) is placed on the dorsal forearm. A constant current output is applied between the two electrodes (10, 70) on the epidermal surface (80). The voltage difference V between the two electrodes is measured and varies from adjacent skin sites as the electrical conductance of the skin changes. The reference electrode (70) may comprise a conductive carbon impregnated silastic pad provided with an insulated metal foil sheet laminated thereto. The metal foil sheet is in electrical contact with a connector element. The reference electrode may further contain adhesive layer laminated to the bottom of the silastic pad provided with a silicon release sheet attached to the adhesive layer. Reference electrode may comprise a carbon-impregnated silastic pad provided with a layer of pharmaceutical electrode gel placed on the bottom of the pad to be positioned against the skin.
FIG. 10(A) depicts the constant current input (I) for each sub-electrode (10a through 10h), numbers 1-8 respectively, as shown in FIG. 9. FIG. 10(B) depicts the voltage output V for each sub-electrode. With reference to FIG. 9 and FIG. 10(B), electrode number (10b), number 2 in FIG. 10(B) is positioned over ulnar nerve (88). As shown in FIG. 10(B), electrode (10b) indicates the position of the ulnar nerve (88) by a decrease in output voltage. Similarly, electrodes (10d) and (10e), numbers 4 and 5 in FIG. 10(B), display a similar output voltage decrease as they are positioned over median nerve (84). Thus, the non-invasive, peripheral nerve mapping device according to the present invention accurately identifies the location of subcutaneous nerves. Voltage minima (conductance maxima) are observed over the ulnar and median nerves (88, 84) at constant current. Sites of decreased skin voltage differentials are mapped and have been shown by nerve stimulator technique, direct dissection and local anesthetic blockage, in animal and human models, to correspond to the location of subcutaneous nerves.
The problem of avoiding metallic interfaces with the skin surface is addressed by the use of water-saturated felt electrodes in U.S. Pat. No. 5,560,372 to Cory and by the use of hydrogels (Jossinet and McAdams, Hydrogel Electrodes in Biosignal Recording. Annual International Conference on the IEEE Engineering in Medicine and Biology Society, 12(4):1490-1491, 1990). The ability to obtain reproducible skin surface conductance and resistance readings allows the recognition of skin surface sites that correspond to underlying peripheral nerves. While this approach circumvents the problems of current density disparities, of the formation of thin oxidation films on the electrodes, and of subsequent back electromotive force, additional problems remain that are associated with the interface between the sampling electrodes and the skin surface.
It is an object of the present invention to provide a sensor system comprising an electrode array and a sensor attachment system for use with an electrical field generating device that can non-invasively detect peripheral nerves.
It is a further object of the present invention to provide a method for detecting peripheral nerves using the aforementioned sensor system.
It is a further object of the present invention to provide for an electrode array, which is flexible, reusable, and suitable for use, either alone or in combination with a sensor attachment system as herein described.
It is a further object of the present invention to provide for a sensor attachment system, comprising conductor islands, which is disposable and suitable for use in combination with an electrode array as herein described.
Further objects and advantages of the invention will be apparent from the following description of the invention.
In satisfaction of the foregoing objects and advantages, the present invention provides an electrode array comprising:
a sheet of electrically non-conductive material having a sensor electrode region, an instrumentation connector region and a flexible stem region mechanically joining the electrode sensor region and the instrumentation connector region;
an electrode array having one or more electrodes, which are disposed within the sensor electrode region,
a connection lead corresponding to each electrode disposed within the instrumentation connector region,
a return lead disposed within the instrumentation connector region,
an electrically conductive connection between each electrode and its corresponding connection lead; and
features on the sensor electrode region for alignment with a skin attachment system, and for alignment with the image displayed by a nerve location device.